Why does the guest operating system show a low CPU frequency?

The CPU frequency displayed in the guest OS is derived from the processor’s Time Stamp Counter (TSC). On latest CPUs, the TSC runs at a fixed, invariant frequency, which is typically set to either the base (nominal) frequency or the maximum all-core turbo frequency guaranteed under sustained AVX workloads — whichever is lower.

For Intel® Xeon processors, these two values often differ significantly. When AVX2/AVX-512 instructions are used, the CPU dynamically reduces its operating frequency to stay within thermal and power limits. However, the TSC frequency remains constant and is based on the worst-case sustained frequency (e.g., under AVX-512 load), not the peak or current performance frequency.

For example, according to Intel specifications for Cascade Lake and newer families:

• Xeon Gold 5220: TSC = 1.4 GHz (vs. nominal 2.20 GHz);
• Xeon Gold 6262V: TSC = 1.1 GHz (vs. nominal 1.9 GHz);
• Xeon Gold 6240: TSC = 1.6 GHz (vs. nominal 2.6 GHz);
• Xeon Gold 6240R: TSC = 1.5 GHz (vs. nominal 2.4 GHz);
• Xeon Gold 6336Y: TSC = 1.6 GHz (vs. nominal 2.4 GHz);
• Xeon Gold 6448Y: TSC = 1.6 GHz (vs. nominal 2.1 GHz);
• Xeon Gold 6548Y: TSC = 1.7 GHz (vs. nominal 2.5 GHz).

Because the TSC runs at this lower, fixed frequency, the value shown in the guest OS does not reflect the actual dynamic clock speed of the CPU cores during operation. The real (effective) frequency changes in real time based on workload, thermal conditions, and instruction set—often running well above the TSC frequency—but the OS still reports the invariant TSC rate.